1. Field of the Invention
The invention relates to a honeycomb structure.
2. Description of the Related Art
A honeycomb catalyst is generally used to convert the exhaust gas of vehicles. The honeycomb catalyst is conventionally manufactured by making a material of a high specific surface area, for example, active alumina, and a catalyst metal, for example, platinum, carried on the surface of an integral cordierite base honeycomb structure of low thermal expansion characteristics. An alkaline earth metal, such as Ba, is carried on the honeycomb structure as a NOx storage agent to treat NOx in an excess oxygen atmosphere of, for example, lean burn engines and diesel engines. The enhanced conversion performance is attained by improving the potential for bringing the exhaust gas into contact with the noble metal catalyst and the NOx storage agent. For this purpose, it is required to increase the specific surface area of the carrier while decreasing the particle size of the noble metal and making the particles of the noble metal highly dispersed. A simple increase in the carried amount of the material having a high specific surface area, for example, active alumina, may, however, only increase the thickness of an alumina layer, while not significantly increasing the contact potential and undesirably heightening a pressure loss. One proposed technique accordingly specifies the shape of cells, the cell density, and the wall thickness to enhance the contact potential (see, for example, JP-A 10-263416). Another known honeycomb structure is manufactured by extrusion molding a material of a high specific surface area with inorganic fibers and an inorganic binder (see, for example, JP-A 5-213681). One joining a honeycomb unit through an adhesive layer so as to enlarge such a honeycomb structure has been known (for example, see DE-A 4341159). The contents of JP-A 10-263416, JP-A 5-213681, and DE 4341159 A are incorporated by reference herein.